The present invention relates to a turbocharger control apparatus and a control method of a turbocharger with a variable nozzle, both for variably controlling an entry port area of an exhaust turbine of the turbocharger so as to enhance warm-up performance of an internal combustion engine.
In a truck in which a diesel engine is mounted, in order to warm up the engine quickly after a start of the engine, a butterfly valve of an exhaust brake apparatus provided in an exhaust pipe of the engine is closed when an accelerator is off (the accelerator is positioned at its initial position) and the engine is idling.
This is because that if the butterfly valve of the exhaust brake apparatus is closed, an exhaust resistance of the engine is increased to increase a load of the engine, so that an amount of the fuel injected to each of cylinders of the diesel engine is increased (so as to keep an idling engine speed constant) and an engine speed for warming up the engine becomes faster.
However, this causes problems that black smoke is generated easily due to a shortage of intake air to be drawn by the diesel engine, and that a noise is generated when the exhaust gas passes through a narrow gap between a periphery of the fully closed butterfly valve and an inner surface of the exhaust pipe.
Thereupon, in order to quicken the warm-up of the engine it has been proposed to use a turbocharger provided at its exhaust gas introducing port with a variable nozzle (It varies the area of an exhaust gas introducing port in accordance with an operation state of the engine.) for supercharging the engine from its low speed.
By this proposal, the exhaust resistance in the exhaust pipe is increased (an exhaust gas pressure is increased) with the variable nozzle of the turbocharger at the time of idling of the engine, as in the above mentioned case that uses the exhaust brake apparatus.
More specifically, as disclosed in Jpn. Utility Model Appln. KOKAI Publication No. 4-33384, when an engine temperature is equal to or less than a predetermined temperature (a border temperature between a cold engine and a hot engine), e.g., 20.degree. C. or less, and the engine is in a light load state, the narrowest nozzle opening degree is selected among a plurality of nozzle opening degrees of the variable nozzle which are normally used when an automobile (vehicle) is running, and otherwise, one of the other nozzle opening degrees is selected in accordance with the operational state of the engine.
In the meantime, it is required to complete the warm-up of the engine as soon as possible so that the automobile can start as soon as possible.
However, the plurality of nozzle opening degrees of the variable nozzle which are normally used when the automobile runs are so set that an appropriate boost pressure can be obtained at any engine speed, i.e., a low engine speed state, an intermediate engine speed state and a high engine speed state, so that the turbocharger can supercharge the engine appropriately in accordance with the running state of the automobile.
As described above, since the plurality of nozzle opening degrees do not include a nozzle opening degree suitable for an extremely low engine speed state such as an idling state, even if the variable nozzle is set at the narrowest nozzle opening degree, a sufficient engine warm-up quickening effect can not be obtained.
That is, the narrowest nozzle opening degree of the conventional variable nozzle set at the time of the warm-up generates a flow rate of the exhaust gas which is most suitable for the low engine speed state, but does not generate a flow rate of the exhaust gas which is most suitable for the extremely low engine speed state such as the idling state and therefore, the exhaust resistance (the exhaust gas pressure) in the exhaust pipe is not sufficiently enhanced. Further, the supercharging operation of the turbocharger is insufficient, an intake of air to the cylinder is insufficient, an increase in heat generated by the engine can not be expected, and a sufficient warm-up quickening effect can not be obtained.
A driver or occupant of the truck frequently takes a nap while the engine is idling and a heater is operating. However, since the heat generated by the engine is low at the time of idling as described above, it is difficult to keep a temperature of the coolant of the engine at a predetermine value and sufficient heating performance can not be maintained when the open air temperature is extremely low.
Thereupon, it is proposed to add a nozzle opening degree suitable for the idling engine speed to the plurality of conventional nozzle opening degrees, the newly added nozzle opening degree being smaller than that for the conventional one suitable for the low engine speed.
However, if the nozzle opening degree of such an extremely small value is newly added, although the sufficient warm-up quickening effect can be obtained, the exhaust turbine tends to rotate excessively just after the warm-up of the engine is completed and the automobile starts to run because at this time the nozzle opening degree for the low engine speed state has not set in place of the newly added nozzle opening degree for idling while an accelerator pedal is depressed to bring the engine into a light loaded state and the flow rate of the exhaust gas is increased.
Therefore, although the warm-up of the engine can be quickened, a durability of the turbocharger may be shortened or excessive air may be flown into the cylinder by the excessive rotation of the exhaust turbine to deteriorate a fuel economy of the engine.